Digital-to-analog converters (“DACs”) are circuits configured to convert digital code words at the input into analog voltages at the output. A digital code word consists of a number N of binary bits (b1, b2 . . . bN). The analog voltage output from the DAC corresponds to a particular binary scaling of a reference voltage VREF, where the binary scaling is related to the digital code word. The minimum step size for the analog output voltage between adjacent code words corresponds to VREF/2N, where N is the number of binary bits in the digital code word.
A resistor ladder works as a large voltage divider with its nodes having incrementally increasing voltage values. A switch matrix selects one or more of the nodes of the resistor ladder to be coupled with the output node of the DAC based on the value of the input digital code word. A resistor ladder DAC (also referred to as resistor string DAC) is a type of DAC that includes a plurality of resistors connected in series between a high reference voltage and a low reference voltage. One or more of the nodes connecting the resistors of the resistor ladder can be selectively coupled with an output node of the DAC in response to the different values of the digital code word at the input. For each unique value of the input digital code word, different ones of the nodes of the resistor ladder are coupled with the output node.
A segmented resistor DAC comprises a DAC that is implemented with both resistor ladders and segmentation where higher-order bits (referred to as most-significant-bits (“MSBs”)) of the input digital code word can be decoded separately from the lower-order bits (referred to as least-significant bits (“LSBs”)). This is done to effectively reduce the amount of resistors needed to generate the output analog voltage values. Previous segmented resistor DAC solutions implemented using resistor ladders are configured to switch resistors in or out of the resistive path based on bit decoding of the input digital code word. This has a number of disadvantages.
First, only a fraction of the resistors in the resistor ladder is utilized for each combination of input bit patterns, while the other resistors are switched out of the resistive path, and are therefore redundant. This results in a significant amount unused resistors and wasted device die area. Further, because conventional solutions utilize different numbers and configurations of resistors for the various different input bit patterns, mismatches often occur in the circuit leading to problems of non-linearity and non-monotonicity.